Downlink (DL) reference signals, which are transmitted from Base Stations (BS) to User Equipment (UE), may be used by UEs to determine a position of the UE. For example, in Observed Time Difference of Arrival (OTDOA) based DL positioning, the mobile station may measure time differences in received signals from a plurality of base stations (e.g. LTE enhanced Node Bs). When positions of the base stations are known, the observed time differences may be used to calculate the location of the UE using multilateration. The terms position and location are used synonymously herein.
In general, UEs may perform measurements of any DL signals to determine position. For example, UEs may perform measurements of Cell-specific Reference Signals (CRS) to determine position. However, DL signals may sometimes suffer from poor from hearability at the UE because of inter-cell interference. For example, when the Signal to Interference and Noise Ratio (SINR) is low, the UE may not detect a sufficient number of base stations thereby detrimentally affecting multilateration. To improve hearability, Positioning Reference Signals (PRS) may be used for location determination. PRS are pseudo-random sequences that avoid collision with CRS signals and overlap with control channels.
In Observed Time Difference of Arrival (OTDOA) based DL positioning, the UE may measure time differences in received signals from a plurality of base stations. When positions of the base stations are available, the observed time differences may be used to calculate the location of the UE. The measured time difference of arrival of the PRS from a reference cell, which can be the serving cell, and one or more neighboring cells is known as the Reference Signal Time Difference (RSTD). Using (i) the RSTD measurements, (ii) the absolute or relative transmission timing of each neighboring cell, and (iii) the known position(s) of BS physical transmitting antennas for the reference and neighboring cells, the UE's position may be determined using multilateration.
Reference Signals (RS) (e.g. PRS and/or CRS) may be used for DL positioning and UE position may be determined when TOA range measurements from 3 or more non-collocated (or geographically distributed) base stations are available at the same time. However, in some situations, one or more neighbor cell RS' may be unavailable (e.g. become too weak to detect) when the serving cell gets stronger. Thus, only one or two non-collocated BS measurements may be available thereby potentially creating a positioning coverage “hole” near base stations, which may make reliable or robust DL positioning challenging. As another example, a sufficient number of RS' may sometimes be unavailable in indoor environments.
Robust DL positioning methods may facilitate UE position determination in instances when position determination is detrimentally impacted by low availability of wireless signals for measurement.